Embodiments of the invention relate generally to magnetic resonance (MR) imaging and, more particularly, to generating MR phase contrast images from a region-of-interest (ROI) near metal.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The use of MR in musculoskeletal (MSK) diagnostics is a rapidly growing field. Arthroplasty is the surgical placement of implants. The population of patients having some form of metal implant is quite large and growing rapidly. MR has significant capabilities in assisting the diagnosis of implant revisions. Using magnetic resonance imaging to assist in clinical diagnostics of MR-compatible arthroplastic implants, however, has proven a fundamentally challenging problem. Most materials that are robust and durable enough to utilize for bone replacements will have magnetic properties that, when placed in a typical B0 magnetic field, induce extraneous fields of amplitude and spatial variation that are large compared to the field offsets utilized in conventional spatial encoding. Accordingly, these materials can introduce distortions in the main magnetic field resulting in an inhomogeneous magnetic field.
MR imaging in the presence of metal hardware has developed increasing importance in view of metal-on-metal implant replacement failure modes that are not detectable via other imaging techniques. The complications from such implants have been attributed to both high rates of wear and hypersensitivity reactions without high wear rates. Unfortunately, conventional magnitude contrast techniques cannot easily differentiate between such soft tissue variations.
Since some of the variation between these tissue constructions involves the presence of high magnetic susceptibility particulate matter, a potential differentiating contrast may be gained from phase analysis of MR images in these regions. However, the substantial Bo perturbations induced by metal render conventional phase-contrast mechanisms useless.
It would therefore be desirable to have a system and method capable of reducing image artifacts near or around implant interfaces to allow for differentiation between and within these classes of local tissue response using MR imaging.